Added the Windows framebuffer implementation.

This actually ends up being faster than Direct3D with a dynamic texture. (???)

src/video/windows/SDL_windowswindow.h

@@ -38,6 +38,8 @@ typedef struct
     SDL_Window *window;
     HWND hwnd;
     HDC hdc;
+    HDC mdc;
+    HBITMAP hbm;
     WNDPROC wndproc;
     SDL_bool created;
     int mouse_pressed;